I keep hearing good things about 7TV and Pulp Alley.
Something I've played and which would do well on all the points you list is 5150:Star Army by Two Hour Wargames.
1. Made by THW, and not a miniatures manufacturer (because why bother with rules sets made by people who use it as a marketing tool to sell figures?)
2. Excellent campaign system, also written to be played co-op or solo without any problem.
3. The game's squad-to-platoon level and includes rules for vehicles as well.
4. Hm. That's a problem in general with 28mm figures, but as long as you use enough terrain...
Any of the rules sets by Ganesha Games (Mutants and Rayguns, Flying Lead, etc.) will fit the bill as well. Good rules sets, circa squad-level games.